In many industries, it is desirable to monitor airborne particles. Toxicoligists, for example, use a photometer to determine the concentration of particles administered to lab animals. Photometers are also used in the field to measure the concentration of airborne particles at a factory, for example. The concentration of dust in the air is also measured using a photometer.
A typical photometer is constructed to focus radiation (e.g., infrared radiation) through an aerosol passage. Optical components such as lenses collimate the radiation into a beam transverse to the flow of the aerosol containing the particles. Radiation scattered by the aerosol particles is detected by a detector such as a photodetector and the amount of scattered radiation detected is proportional to the concentration of the particles in the aerosol stream.
The particles, however, can contaminate and obscure the optical components of the photometer resulting in erroneous readings of the scattered radiation. As a result, the photometer must be recalibrated before each use which is a nuisance especially if the photometer is used often. Cleaning the optical lenses cannot generally be accomplished by the user.
Those skilled in the art have attempted to purge the lenses with air and have also attempted to maintain a positive pressure in the area of the lenses. Purge air at a low flow rate, however, is not effective at cleaning the lenses and purge air at a flow rate which might clean the lenses dilutes the aerosol stream resulting in erroneous particle concentration readings.